Preparation of alkyl carboxylates

ABSTRACT

A process for preparation of alkyl carboxylate mixtures enriched in alpha-methylalkyl carboxylate compounds, by reaction of an olefin and a carboxylic acid compound in the presence of a particular type of zeolite catalyst. The zeolites are characterized by a silica to alumina mole ratio of at least 12 and a constraint index of 1 to 12. Zeolites ZSM-5 and ZSM-12 are particularly preferred.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of the copending applicationof Lewis B. Young, said application having Ser. No. 218,148, filed Dec.19, 1980, now U.S. Pat. No. 4,365,084.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is concerned with the preparation of alkyl carboxylatesmixtures which are enriched in alpha-methylalkyl carboxylates. Suchmixtures are prepared by reaction of carboxylic acids with olefiniccompounds in the presence of a particular type of crystalline zeolitecatalyst.

2. Description of the Prior Art

The addition of carboxylic acids to olefins to make esters is known. Thechemical literature describes the use of Lewis acid catalysts, such asBF₃, to promote the reaction. However, particularly in the case ofinternal olefins, the reaction will result in addition of the carboxylicacid to both ends of the double bond, thereby giving rise to a mixtureof carboxylate products. Mineral acids (e.g., H₂ SO₄) are also reportedto catalyze the reaction, but the result is much the same, i.e.,non-selective addition of the carboxylic acid to either side of thecarbon-carbon double bond. Trevillyan, U.S. Pat. No. 3,492,341; issuedJan. 27, 1970 discloses preparation of alkyl carboxylate esters byreacting carboxylic acids with 1- or 2-monoolefins over a mordenitezeolite catalyst.

In the past, the only known reaction route to directly produce alkylcarboxylate mixtures enriched in alpha-methylalkyl carboxylates hasrequired the utilization of expensive alpha-olefins. Reaction withinternal or mixed olefins using common acid catalysts has necessitatedphysical separation of the isomeric variants of the alkyl carboxylateproduct by other means, such as distillation, in order to isolate aproduct which is rich in the α-methylalkyl carboxylate.

Alkyl esters of carboxylic acids are useful as solvents, plasticizersand chemical intermediates. Alpha-methylalkyl carboxylates areparticularly useful for making secondary alcohols with hydroxylattachment at the 2-carbon. By utilization of the herein disclosedmethod, alpha-methylalkyl carboxylate enriched products normally derivedfrom pure alpha-olefins can now be prepared from less expensive linearolefin mixtures.

SUMMARY OF THE INVENTION

It has now been discovered that certain zeolite materials may beutilized to promote reaction between olefins and carboxylic acids toproduce an alkyl carboxylate reaction product enriched inalpha-methylalkyl carboxylate isomers. In a particularly preferredembodiment, alpha-methylalkyl carboxylates may be prepared as the majorproduct from the reaction of carboxylic acids and internal linearolefins or olefin mixtures containing internal olefins. Specifically,the alpha-methylalkyl carboxylates contemplated herein as the principalalkyl carboxylate produced are those described by the formula: ##STR1##wherein:

R₁ =alkyl, aryl, haloalkyl or hydrogen, and preferably=alkyl of 1 to 10carbons; and

R₂ =C₁ -C₂₀ alkyl, heteroalkyl or cycloalkyl, and preferably=C₄ -C₁₈alkyl.

The method comprises reacting a carboxylic acid having the formula##STR2## R₁ being described above, with an olefin having from 3 to about20 carbon atoms. Substantially any linear, slightly branched, cyclic orheteroatom-substituted olefin may be employed, regardless of theposition of the carbon-carbon double bond. However, linear olefins arepreferred. In a particularly preferred embodiment, olefin mixtures areutilized wherein such mixtures contain at least 25 mole percent of a C₆-C₂₀ olefin having no unsaturation at the site of the No. 2 carbon atomof the olefin.

A wide range of temperature and pressure conditions are found to beconducive to the reaction, which may be successfully carried out at 25°C. to 600° C. and 10⁴ Pa to 10⁷ Pa (0.1 to 100 atmospheres) pressure.Temperatures of between 75° C. and 400° C. are preferred, as arepressures of 10⁵ Pa to 40×10⁵ Pa (1 to 40 atmospheres). The reaction maybe usefully carried out in either the liquid or vapor phase, although itmay be found preferable to employ liquid phase reaction.

The particular type of zeolite catalysts which are found to promote thisnovel, selective addition reaction are characterized by their opencrystal structure having channels or networks of pores which providerestricted passageways for entry and egress of the organic reactants.These zeolites may be identified by their characteristic ConstraintIndex of 1 to 12 and their relatively high silica to alumina ratios ofat least 12. There are several known members of the class, such aszeolites ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48.Zeolite ZSM-12 is preferred.

DESCRIPTION OF SPECIFIC EMBODIMENTS

A one-step process has now been found for the manufacture of alkylcarboxylates from carboxylic acids and olefins, with especiallydesirable selectivity to production of an alpha-methylalkylcarboxylate-enriched product. By utilization of the particular type ofzeolite catalyst described hereinafter, it now becomes possible to reactcarboxylic acids with olefins having the carbon-carbon double bond insubstantially any position in the molecule and produce an adduct whereinthe carboxylate has attached principally at the #2 carbon of the olefinmolecule. This is in striking contrast to the reaction product resultingfrom utilization of Lewis acid and mineral acid catalysts wherein thecarboxylate would attach at the carbons on either side of the doublebond and, unless the site of the unsaturation included the #2 carbonatom, the resultant yield of alpha-methylalkyl carboxylates in the alkylcarboxylate reaction product would comprise no more than a minorbyproduct.

The carboxylic acids useful in the process of the present invention arepreferably alkyl carboxylic acids having from 1 to about 10 carbon atomstherein. Included within this group are, for example, formic acid,acetic acid, propionic acid, butyric acid and hexanoic acid. Slightlybranched alkyl carboxylic acids are also useful, such as, for instance,isobutyric acid. Haloalkyl carboxylic acids, such as chloroacetic acid,fluoroacetic acid and trifluoroacetic acid may be employed. Also, arylcarboxylic acids will be found desirable in some instances, includingbenzoic acid, para-toluic acid and parachlorobenzoic acid. Forcommercially viable applications, it is expected that acetic acid willbe found particularly desirable.

Olefins suitable for manufacture of alpha-methylalkyl carboxylatemixtures as described herein are not limited to alpha-olefins. Rather,it has been found that substantially any olefinic hydrocarbons may beemployed without regard to the location of the site of unsaturation.Mixed isomers of a given olefin are particularly desirable due to theirready availability and relatively low cost. Linear C₃ -C₂₀ olefins areespecially preferred, but slightly branched olefins may also beemployed. Some non-limiting examples include propylene, butene, octene,dodecene, hexadecene and 1-methylnonene. Especially prepared olefinicreactants are olefin mixtures containing at least 25% of a C₆ to C₂₀olefin having no unsaturation at the site of the No. 2 carbon atomthereof. It is especially surprising that internal olefins of this typecan comprise a substantial part or even all of the olefinic reactant andstill have the resulting alkyl carboxylate product enriched in thealpha-methylalkyl carboxylate.

The crystalline zeolites utilized herein are members of a particularclass of zeolitic materials which exhibit unusual properties. Althoughthese zeolites have unusually low alumina contents, i.e. high silica toalumina mole ratios, they are very active even when the silica toalumina mole ratio exceeds 30. The activity is surprising sincecatalytic activity is generally attributed to framework aluminum atomsand/or cations associated with these aluminum atoms. These zeolitesretain their crystallinity for long periods in spite of the presence ofsteam at high temperature which induces irreversible collapse of theframework of other zeolites, e.g. of the X and A type. Furthermore,carbonaceous deposits, when formed, may be removed by burning at higherthan usual temperatures to restore activity. These zeolites, used ascatalysts, generally have low coke-forming activity and therefore areconducive to long times on stream between regenerations by burningcarbonaceous deposits with oxygen-containing gas such as air.

An important characteristic of the crystal structure of this particularclass of zeolites is that it provides a selective constrained access toand egress from the intracrystalline free space by virtue of having aneffective pore size intermediate between the small pore Linde A and thelarge pore Linde X, i.e. the pore windows of the structure are of abouta size such as would be provided by 10-membered rings of silicon atomsinterconnected by oxygen atoms. It is to be understood, of course, thatthese rings are those formed by the regular disposition of thetetrahedra making up the anionic framework of the crystalline zeolite,the oxygen atoms themselves being bonded to the silicon (or aluminum,etc.) atoms at the centers of the tetrahedra.

The silica to alumina mole ratio referred to may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although zeolites with silica to alumina moleratios of at least 12 are useful, it is preferred in some instances touse zeolites having substantially higher silica/alumina ratios, e.g.1600 and above. In addition, zeolites as otherwise characterized hereinbut which are substantially free of aluminum, that is zeolites havingsilica to alumina mole ratios of up to infinity, are found to be usefuland even preferable in some instances. Such "high silica" or "highlysiliceous" zeolites are intended to be included within this description.Also included within this definition are substantially pure silicaanalogs of the useful zeolites described herein, that is to say thosezeolites having no measurable amount of aluminum (silica to alumina moleratio of infinity) but which otherwise embody the characteristicsdisclosed.

The special class of zeolites, after activation, acquire anintracrystalline sorption capacity for normal hexane which is greaterthan that for water, i.e. they exhibit "hydrophobic" properties. Thishydrophobic character can be used to advantage in some applications.

The particular class of zeolites useful herein have an effective poresize such as to freely sorb normal hexane. In addition, the structuremust provide constrained access to larger molecules. It is sometimespossible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of silicon and aluminum atoms,then access by molecules of larger cross-section than normal hexane isexcluded and the zeolite is not of the desired type. Windows of10-membered rings are preferred, although in some instances excessivepuckering of the rings or pore blockage may render these zeolitesineffective.

Although 12-membered rings in theory would not offer sufficientconstraint to produce advantageous conversions, it is noted that thepuckered 12-ring structure of TMA offretite does show some constrainedaccess. Other 12-ring structures may exist which may be operative forother reasons and, therefore, it is not the present intention toentirely judge the usefulness of a particular zeolite solely fromtheoretical structural considerations.

Rather than attempt to judge from crystal structure whether or not azeolite possesses the necessary constrained access to molecules oflarger cross-section than normal paraffins, a simple determination ofthe "Constraint Index" as herein defined may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a sample of zeolite at atmospheric pressureaccording to the following procedure. A sample of the zeolite, in theform of pellets or extrudate, is crushed to a particle size about thatof coarse sand and mounted in a glass tube. Prior to testing, thezeolite is treated with a stream of air at 540° C. for at least 15minutes. The zeolite is then flushed with helium and the temperature isadjusted between 290° C. and 510° C. to give an overall conversion ofbetween 10% and 60%. The mixture of hydrocarbons is passed at 1 liquidhourly space velocity (i.e., 1 volume of liquid hydrocarbon per volumeof zeolite per hour) over the zeolite with a helium dilution to give ahelium to (total) hydrocarbon mole ratio of 4:1. After 20 minutes onstream, a sample of the effluent is taken and analyzed, mostconveniently by gas chromatography, to determine the fraction remainingunchanged for each of the two hydrocarbons.

While the above experimental procedure will enable one to achieve thedesired overall conversion of 10 to 60% for most zeolite samples andrepresents preferred conditions, it may occasionally be necessary to usesomewhat more severe conditions for samples of very low activity, suchas those having an exceptionally high silica to alumina mole ratio. Inthose instances, a temperature of up to about 540° C. and a liquidhourly space velocity of less than one, such as 0.1 or less, can beemployed in order to achieve a minimum total conversion of about 10%.

The "Constraint Index" is calculated as follows: ##EQU1##

The Constraint Index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Zeolites suitable for the presentinvention are those having a Constraint Index of 1 to 12. ConstraintIndex (CI) values for some typical materials are:

    ______________________________________                                                          C.I.                                                        ______________________________________                                        ZSM-4               0.5                                                       ZSM-5               8.3                                                       ZSM-11              8.7                                                       ZSM-12              2                                                         ZSM-23              9.1                                                       ZSM-35              4.5                                                       ZSM-38              2                                                         ZSM-48              3.4                                                       TMA Offretite       3.7                                                       Clinoptilolite      3.4                                                       Beta                0.6                                                       H--Zeolon (mordenite)                                                                             0.4                                                       REY                 0.4                                                       Amorphous Silica-Alumina                                                                          0.6                                                       Erionite            38                                                        ______________________________________                                    

The above-described Constraint Index is an important and even criticaldefinition of those zeolites which are useful in the instant invention.The very nature of this parameter and the recited technique by which itis determined, however, admit of the possibility that a given zeolitecan be tested under somewhat different conditions and thereby exhibitdifferent Constraint Indices. Constraint Index seems to vary somewhatwith severity of operation (conversion) and the presence or absence ofbinders. Likewise, other variables such as crystal size of the zeolite,the presence of occluded contaminants, etc., may affect the constraintindex. Therefore, it will be appreciated that it may be possible to soselect test conditions as to establish more than one value in the rangeof 1 to 12 for the Constraint Index of a particular zeolite. Such azeolite exhibits the constrained access as herein defined and is to beregarded as having a Constraint Index in the range of 1 to 12. Alsocontemplated herein as having a Constraint Index in the range of 1 to 12and therefore within the scope of the defined class of highly siliceouszeolites are those zeolites which, when tested under two or more sets ofconditions within the above-specified ranges of temperature andconversion, produce a value of the Constraint Index slightly less than1, e.g. 0.9, or somewhat greater than 12, e.g. 14 or 15, with at leastone other value within the range of 1 to 12. Thus, it should beunderstood that the Constraint Index value as used herein is aninclusive rather than an exclusive value. That is, a crystalline zeolitewhen identified by any combination of conditions within the testingdefinition set forth herein as having a Constraint Index in the range of1 to 12 is intended to be included in the instant zeolite definitionwhether or not the same identical zeolite, when tested under other ofthe defined conditions, may give a Constraint Index value outside of therange of 1 to 12.

The particular class of zeolites defined herein is exemplified by ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, and other similarmaterials.

ZSM-5 is described in greater detail in U.S. Pat. No. 3,702,886 and U.S.Pat. No. Re. 29,948. The entire descriptions contained within thosepatents, particularly the X-ray diffraction pattern of therein disclosedZSM-5, are incorporated herein by reference.

ZSM-11 is described in U.S. Pat. No. 3,709,979. That description, and inparticular the X-ray diffraction pattern of said ZSM-11, is incorporatedherein by reference.

ZSM-12 is described in U.S. Pat. No. 3,832,449. That description, and inparticular the X-ray diffraction pattern disclosed therein, isincorporated herein by reference.

ZSM-23 is described in U.S. Pat. No. 4,076,842. The entire contentthereof, particularly the specification of the X-ray diffraction patternof the disclosed zeolite, is incorporated herein by reference.

ZSM-35 is described in U.S. Pat. No. 4,016,245. The description of thatzeolite, and particularly the X-ray diffraction pattern thereof, isincorporated herein by reference.

ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859. Thedescription of that zeolite, and particularly the specified X-raydiffraction pattern thereof, is incorporated herein by reference.

ZSM-48 is more particularly described in European Patent Application No.80 300,463, published Sept. 3, 1980 as Publication No. 0015132, thecontent of which is incorporated herein by reference.

It is to be understood that by incorporating by reference the foregoingpatents to describe examples of specific members of the zeolite classwith greater particularity, it is intended that identification of thetherein disclosed crystalline zeolites be resolved on the basis of theirrespective X-ray diffraction patterns. As discussed above, the presentinvention contemplates utilization of such catalysts wherein the moleratio of silica to alumina is essentially unbounded. The incorporationof the identified patents should therefore not be construed as limitingthe disclosed crystalline zeolites to those having the specificsilica-alumina mole ratios discussed therein, it now being known thatsuch zeolites may be substantially aluminum-free and yet, having thesame crystal structure as the disclosed materials, may be useful or evenpreferred in some applications. It is the crystal structure, asidentified by the X-ray diffraction "fingerprint", which establishes theidentity of the specific crystalline zeolite material.

The specific zeolites described, when prepared in the presence oforganic cations, are substantially catalytically inactive, possiblybecause the intra-crystalline free space is occupied by organic cationsfrom the forming solution. They may be activated by heating in an inertatmosphere at 540° C. for one hour, for example, followed by baseexchange with ammonium salts followed by calcination at 540° C. in air.The presence of organic cations in the forming solution may not beabsolutely essential to the formation of this type zeolite; however, thepresence of these cations does appear to favor the formation of thisspecial class of zeolite. More generally, it is desirable to activatethis type catalyst by base exchange with ammonium salts followed bycalcination in air at about 540° C. for from about 15 minutes to about24 hours.

Natural zeolites may sometimes be converted to zeolite structures of theclass herein identified by various activation procedures and othertreatments such as base exchange, steaming, alumina extraction andcalcination, alone or in combinations. Natural minerals which may be sotreated include ferrierite, brewsterite, stilbite, dachiardite,epistilbite, heulandite, and clinoptilolite.

The preferred crystalline zeolites for utilization herein include ZSM-5,ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48, with ZSM-5 and ZSM-12being particularly preferred.

In a preferred aspect of this invention, the zeolites hereof areselected as those providing among other things a crystal frameworkdensity, in the dry hydrogen form, of not less than about 1.6 grams percubic centimeter. It has been found that zeolites which satisfy allthree of the discussed criteria are most desired for several reasons.When hydrocarbon products or by-products are catalytically formed, forexample, such zeolites tend to maximize the production of gasolineboiling range hydrocarbon products. Therefore, the preferred zeolitesuseful with respect to this invention are those having a ConstraintIndex as defined above of about 1 to about 12, a silica to alumina moleratio of at least about 12 and a dried crystal density of not less thanabout 1.6 grams per cubic centimeter. The dry density for knownstructures may be calculated from the number of silicon plus aluminumatoms per 1000 cubic Angstroms, as given, e.g., on Page 19 of thearticle Zeolite Structure by W. M. Meier. This paper, the entirecontents of which are incorporated herein by reference, is included inProceedings of the Conference on Molecular Sieves, (London, April 1967)published by the Society of Chemical Industry, London, 1968.

When the crystal structure is unknown, the crystal framework density maybe determined by classical pycnometer techniques. For example, it may bedetermined by immersing the dry hydrogen form of the zeolite in anorganic solvent which is not sorbed by the crystal. Or, the crystaldensity may be determined by mercury porosimetry, since mercury willfill the interstices between crystals but will not penetrate theintracrystalline free space.

It is possible that the unusual sustained activity and stability of thisspecial class of zeolites is associated with its high crystal anionicframework density of not less than about 1.6 grams per cubic centimeter.This high density must necessarily be associated with a relatively smallamount of free space within the crystal, which might be expected toresult in more stable structures. This free space, however, is importantas the locus of catalytic activity.

Crystal framework densities of some typical zeolites, including somewhich are not within the purview of this invention, are:

    ______________________________________                                                      Void     Framework                                                            Volume   Density                                                ______________________________________                                        Ferrierite      0.28   cc/cc     1.76 g/cc                                    Mordenite       .28              1.7                                          ZSM-5, -11      .29              1.79                                         ZSM-12          --               1.8                                          ZSM-23          --               2.0                                          Dachiardite     .32              1.72                                         L               .32              1.61                                         Clinoptilolite  .34              1.71                                         Laumontite      .34              1.77                                         ZSM-4 (Omega)   .38              1.65                                         Heulandite      .39              1.69                                         P               .41              1.57                                         Offretite       .40              1.55                                         Levynite        .40              1.54                                         Erionite        .35              1.51                                         Gmelinite       .44              1.46                                         Chabazite       .47              1.45                                         A               .5               1.3                                          Y               .48              1.27                                         ______________________________________                                    

When synthesized in the alkali metal form, the zeolite is convenientlyconverted to the hydrogen form, generally by intermediate formation ofthe ammonium form as a result of ammonium ion exchange and calcinationof the ammonium form to yield the hydrogen form. In addition to thehydrogen form, other forms of the zeolite may be employed wherein theoriginal alkali metal has been reduced to less than about 50 percent byweight of the original alkali metal contained in the zeolite assynthesized, usually 0.5 percent by weight or less. Thus, the originalalkali metal of the zeolite may be replaced by ion exchange with othersuitable metal cations of Groups I through VIII of the Periodic Table,including, by way of example, nickel, copper, zinc, palladium, calciumor rare earth metals.

In practicing the alkyl carboxylate production process of the presentinvention, it may be useful to incorporate the above-describedcrystalline zeolite with a matrix comprising another material resistantto the temperature and other conditions employed in the process. Suchmatrix material is useful as a binder and imparts greater resistance tothe catalyst for the severe temperature, pressure and reactant feedstream velocity conditions encountered in some processes.

Useful matrix materials include both synthetic and naturally occurringsubstances, as well as inorganic materials such as clay, silica and/ormetal oxides. The latter may be either naturally occurring or in theform of gelatinous precipitates or gels including mixtures of silica andmetal oxides. Naturally occurring clays which can be composited with thezeolite include those of the montmorillonite and kaolin families, whichfamilies include the sub-bentonites and the kaolins commonly known asDixie, McNamee-Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the zeolites employed herein maybe composited with a porous matrix material, such as alumina,silica-alumina, silica-magnesia, silica-zirconia, silica-thoria,silica-beryllia, and silica-titania, as well as ternary compositions,such as silica-alumina-thoria, silica-alumina-zirconia,silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may bein the form of a cogel. The relative proportions of zeolite componentand inorganic oxide gel matrix, on an anhydrous basis, may vary widelywith the zeolite content ranging from between about 1 to about 99percent by weight and more usually in the range of about 5 to about 80percent by weight of the dry composite.

A useful modifying treatment entails steaming of the zeolite by contactwith an atmosphere containing from about 5 to about 100 percent steam ata temperature of from about 250° C. to 1000° C. Steaming may last for aperiod of between about 0.25 and about 100 hours and may be conducted atpressures ranging from sub-atmospheric to several hundred atmospheres toreduce the alpha value of the zeolite to less than 500, and preferablyless than 20, but greater than zero.

The process of this invention is carried out such that the organicreactants, i.e., the carboxylic acid and the olefinic reactant, arebrought into contact with the particular type of zeolite materialdescribed herein in a suitable reaction zone under alkyl carboxylateester-forming reaction conditions. Such conditions include a temperaturewhich is elevated to a level conducive to the addition reaction.Suitable temperatures are from about 25° C. to about 600° C., buttemperatures of between 75° C. and 400° C. are preferred. The reactionzone will preferably be pressurized to approximately 10⁵ Pa to 40×10⁵ Pa(1 to 40 atmospheres) pressure, but pressures falling within the rangeof 10⁴ Pa to 10⁷ Pa (0.1 to 100 atmospheres) will be found to beutilizable.

The ester-forming process described herein may be carried out as abatch-type, semi-continuous or continuous operation utilizing a fixed ormoving bed catalyst system. A preferred embodiment entails use of acatalyst zone wherein the organic charge is passed concurrently orcountercurrently through a moving bed of particle-form catalyst. Thelatter, after use, is conducted to a regeneration zone where coke isburned from the catalyst in an oxygen-containing atmosphere (such asair) at elevated temperature, after which the regenerated catalyst isrecycled to the conversion zone for further contact with the organicreactants.

By utilization of the particular type of zeolite catalyst and reactionconditions described hereinbefore, it now becomes possible to reactcarboxylic acids with olefinic hydrocarbons which have the carbon-carbondouble bond in substantially any position in the molecule and tonevertheless selectively produce an alkyl carboxylate reaction productenriched in the alpha-methylalkyl carboxylate isomer. For purposes ofthe present invention, the alkyl carboxylate product is enriched inalpha-methylalkyl carboxylate when the alpha-methylalkyl isomercomprises at least 40% of the total alkyl carboxylate product.Preferably the alpha-methylalkyl isomer comprises at least 50% or evenat least 60% of the total alkyl carboxylate product.

The following examples are provided to illustrate the process of thisinvention and to aid those in the art in the understanding thereof, butclearly should not be taken as presenting undue limitations thereon:

EXAMPLE 1

One gram of HZSM-12 zeolite (SiO₂ /Al₂ O₃ mole ratio 70) was placed in a300 cc stainless steel autoclave equipped with a magnetically drivenstirrer. The catalyst had been ground to a powder and calcined prior touse. Added 100 ml of a mixture of acetic acid and 1-octene (molarratio=4/1) and heated to temperature. Samples were withdrawnperiodically through a dip tube and analyzed. Results are summarized inTable I.

                  TABLE I                                                         ______________________________________                                        REACTION OF 1-OCTENE AND ACETIC ACID                                          OVER HZSM-12                                                                                                   C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        3.5 hr 150° C.                                                                         160 psig  5.9 wt%                                                                              96.9% 2.9% 0.2%                              5.6 hr 200° C.                                                                         240 psig 23.9 wt %                                                                             94.1% 5.5% 0.4%                              73.2 hr                                                                              200° C.                                                                         230 psig 32.2 wt %                                                                             91.4% 8.1% 0.5%                              ______________________________________                                    

The 1-octene and acetic acid have been converted to primarily 2-octylacetate. The 3- and 4-octyl acetates were produced as minor byproductsand no 1-octyl acetate was formed.

EXAMPLE 2

2-Octene and acetic acid were reacted in the same manner as inExample 1. The catalyst was another sample of the same HZSM-12 zeoliteand the 2-octene reactant was a mixture of the cis and trans isomers.The results are shown in Table II.

                  TABLE II                                                        ______________________________________                                        REACTION OF 2-OCTENE AND ACETIC ACID                                          OVER HZSM-12                                                                                                   C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distibution                                  Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        0.8 hr 150° C.                                                                         175 psig  0.4 wt %                                                                             77% 23% --                                   1.4 hr 200° C.                                                                         245 psig  6.9 wt %                                                                             79% 19% 1.7%                                 2.3 hr 200° C.                                                                         245 psig 12.9 wt %                                                                             79% 20% 0.6%                                 3.3 hr 200° C.                                                                         235 psig 18.3 wt %                                                                             78% 22% 0.7%                                 4.3 hr 225° C.                                                                         300 psig 22.0 wt %                                                                             75% 24% 1.0%                                 6.3 hr 225° C.                                                                         300 psig 24.1 wt %                                                                             69% 28% 2.2%                                 ______________________________________                                    

The use of HZSM-12 zeolite to promote the reaction is seen tosignificantly alter the isomeric product distribution from that whichwould normally be expected. Using the same reactor system and procedure,comparative runs were made with other catalysts as follows:

EXAMPLE 3

2 Octene and acetic acid were reacted over 1.0 g of HZSM-5 zeolite. Theconditions of reaction were the same as in Example 2 and the results aresummarized in Table III.

                  TABLE III                                                       ______________________________________                                        REACTION OF 2-OCTENE AND ACETIC ACID                                          OVER HZSM-5                                                                                                    C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        2.0 hr 200° C.                                                                         290 psig  3.6 wt %                                                                             80% 17%  3%                                  2.8 hr 250° C.                                                                         460 psig  9.0 wt %                                                                             70% 23%  7%                                  4.0 hr 250° C.                                                                         445 psig 10.4 wt %                                                                             55% 30% 16%                                  6.1 hr 250° C.                                                                         440 psig 13.0 wt %                                                                             46% 32% 23%                                  ______________________________________                                    

EXAMPLE 4

Amorphous silica-alumina: SiO₂ /Al₂ O₃ =90/10.

EXAMPLE 5

Zeolite REY.

EXAMPLE 6

A Lewis Acid catalyst: BF₃.ET₂ O.

A mixture of acetic acid and 2-octene (molar ratio 4:1, respectively)was mixed with a small amount of boron trifluoride etherate catalyst andthe mixture heated to 90° C. on a steam bath. Samples were removed andanalyzed at 1.2 and 2.7 hours.

The reactions of Examples 4-6 are summarized in Table IV and acomparison with the HZSM-5 and HZSM-12 zeolite is presented in Table V.

                  TABLE IV                                                        ______________________________________                                        REACTION OF 2-OCTENE AND ACETIC ACID                                                                           C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        Catalyst: BF.sub.3 Et.sub.2 O                                                 1.2 hr  90° C.                                                                          0 psig  37.3 wt %                                                                             52% 45% 3%                                   2.7 hr  90° C.                                                                          0 psig  57.0 wt %                                                                             52% 44% 4%                                   Catalyst: amorphous SiO.sub.2 /Al.sub.2 O.sub.3                               0.6 hr 250° C.                                                                         400 psig  5.0 wt %                                                                             58% 37% 5%                                   1.8 hr 250° C.                                                                         400 psig 11.1 wt %                                                                             59% 36% 5%                                   17.5 hr                                                                              250° C.                                                                         400 psig 17.4 wt %                                                                             62% 30% 8%                                   Catalyst: REY                                                                 2.2 hr 200° C.                                                                         215 psig  1.9 wt %                                                                             59% 37% 4%                                   3.2 hr 250° C.                                                                         380 psig  9.5 wt %                                                                             60% 35% 5%                                   3.9 hr 250° C.                                                                         380 psig 14.5 wt %                                                                             59% 34% 6%                                   5.0 hr 250° C.                                                                         375 psig 16.6 wt %                                                                              59% 31% 10%                                 ______________________________________                                    

                  TABLE V                                                         ______________________________________                                        CATALYST COMPARISONS                                                                                            Ratio:                                                                        2C.sub.8 OAc                                Catalyst                                                                              2-C.sub.8 OAc                                                                           3-C.sub.8 OAc                                                                          4-C.sub.8 OAc                                                                        3 + 4-C.sub.8 OAc                           ______________________________________                                        HZSM-5  80%       17%      3%     4.0                                         HZSM-12 79%       20%      0.6%   3.4                                         BF.sub.3.ET.sub.2 O                                                                   52%       44%      4%     1.1                                         SiO.sub.2 /Al.sub.2 O.sub.3                                                           62%       30%      8%     1.6                                         REY     60%       35%      5%     1.5                                         ______________________________________                                    

It will be clearly seen from the data that the proportion of 2-isomerproduced is significantly improved by utilization of HZSM-5 and HZSM-12zeolites to catalyze the reaction. Similar improvement can be expectedfrom the other zeolites encompassed within the herein described class.

EXAMPLE 7

To demonstrate even more dramatically the novel selectivity of theherein disclosed process, an olefin having the double bond in a moreinternal location--i.e., trans-4-octene--was reacted with acetic acid inthe presence of HZSM-12. The reaction mixture contained a 10/1 molarratio of HOAc to trans-4-octene. The results are given in Table VI.

                  TABLE VI                                                        ______________________________________                                        REACTION OF 4-OCTENE AND ACETIC ACID                                          OVER HZSM-12                                                                                                   C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        1.3 hr 150° C.                                                                         170 psig  1.5 wt %                                                                             61% 24% 15%                                  2.1 hr 200° C.                                                                         225 psig 10.5 wt %                                                                             60% 30% 10%                                  2.8 hr 200° C.                                                                         225 psig 17.3 wt %                                                                             58% 32% 10%                                  3.6 hr 200° C.                                                                         225 psig 24.3 wt %                                                                             57% 32% 11%                                  5.6 hr 200° C.                                                                         225 psig 27.4 wt %                                                                             54% 33% 12%                                  ______________________________________                                    

EXAMPLES 8-10

Using the same reaction mixture as Example 7, comparative reactions werecarried out with a Lewis acid catalyst (BF₃ Et₂ O) as well withamorphous silica-alumina and zeolite REY. Results are shown in TableVII. A direct comparison of the isomeric distribution of the octylactate product with that resulting from ZSM-12 is provided in TableVIII.

                  TABLE VII                                                       ______________________________________                                        REACTION OF 4-OCTENE AND ACETIC ACID                                                                           C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2 / 3 / 4                                    ______________________________________                                        Catalyst: BF.sub.3 Et.sub.2 O                                                 0.6 hr  90° C.                                                                          0 psig  9.0 wt %                                                                              1%  5% 94%                                   4.3 hr  90° C.                                                                          0 psig  65.7 wt %                                                                             8% 18% 75%                                   Catalyst: amorphous SiO.sub.2 /Al.sub.2 O.sub.3                               2.3 hr 250° C.                                                                         390 psig 3.5 wt %                                                                              5% 10% 85%                                   3.5 hr 250° C.                                                                         390 psig 6.9 wt %                                                                              5% 10% 85%                                   4.9 hr 250° C.                                                                         390 psig 8.4 wt %                                                                              6% 11% 83%                                   Catalyst: REY                                                                 1.6 hr 200° C.                                                                         225 psig 4.1 wt %                                                                              4%  7% 89%                                   2.1 hr 250° C.                                                                         375 psig 7.1 wt %                                                                              4% 10% 86%                                   3.6 hr 250° C.                                                                         390 psig 11.5 wt %                                                                             10% 17% 73%                                  ______________________________________                                    

                  TABLE VIII                                                      ______________________________________                                        CATALYST COMPARISONS                                                                                             Ratio:                                                                        2C.sub.8 OAc                               Catalyst                                                                              2-C.sub.8 OAc                                                                           3-C.sub.8 OAc                                                                          4-C.sub.8 OAc                                                                         3 + 4-C.sub.8 OAc                          ______________________________________                                        HZSM-12 60%       30%      10%     1.50                                       BF.sub.3 Et.sub.2 O                                                                    8%       18%      75%     0.09                                       SiO.sub.2 /Al.sub.2 O.sub.3                                                            6%       11%      83%     0.06                                       REY     10%       17%      73%     0.11                                       ______________________________________                                    

As the comparisons of Table VIII show, conventional catalysts and thelarge zeolite, REY, all give rise to the product arising from additionof the carboxylic acid to the double bond. HZSM-12, however, selectivelyyields 2-octyl acetate as the major product.

EXAMPLE 11

A mixed octene sample was utilized to demonstrate the unique selectivityof the process with an isomeric mixture of olefins. The octene consistedof 25% 1-ocetene, 25% t-2-octene, 25% t-3-octene and 25% t-4-octene.Using the same autoclave procedure as above, acetic acid and the mixedoctenes (in a molar ratio of 8:1) were reacted over HZSM-12 zeolite. Theresults are shown in Table IX.

                  TABLE IX                                                        ______________________________________                                        REACTION OF MIXED OCTENES AND ACETIC ACID                                     OVER HZSM-12                                                                                                   C.sub.8 H.sub.17 OAc Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OAc                                                                           2  / 3 / 4                                   ______________________________________                                        0.5 hr 150° C.                                                                         155 psig 10.5 wt %                                                                             88% 11% 1%                                   1.75 hr                                                                              150° C.                                                                         140 psig 14.1 wt %                                                                             86% 13% 1%                                   2.75 hr                                                                              150° C.                                                                         140 psig 17.0 wt %                                                                             86% 13% 1%                                   3.7 hr 200° C.                                                                         185 psig 22.5 wt %                                                                             78% 20% 2%                                   ______________________________________                                    

Assuming an equal probability of addition of the acid to both ends ofthe double bond (except no addition to the terminal position), one wouldnormally expect an octylacetate isomer distribution of 37.5% 2-isomer,25% 3-isomer and 37.5% 4-isomer. The actual result, utilizing theprocess of this invention, indicates a very high level of selectivity ofthe desired 2-isomer.

EXAMPLE 12

Using the same reaction method, 4-octene and propionic acid were reactedover 1.0 g of HZSM-12. The reaction mixture consisted of 100 ml of an8:1 molar ratio mixture of the acid to the olefin. The results are shownin Table X.

                  TABLE X                                                         ______________________________________                                        REACTION OF 4-OCTENE AND PROPIONIC ACID                                       OVER HZSM-12                                                                                               C.sub.8 H.sub.17 OAc Isomer                      Reaction                     Distribution                                     Time     Temperature                                                                              Pressure 2 / 3 / 4                                        ______________________________________                                        1.0 hr   150° C.                                                                           150 psig 62% 16% 22%                                      2.4 hr   200° C.                                                                           195 psig 59% 21% 20%                                      ______________________________________                                    

EXAMPLE 13

Isobutyric acid and t-4-octene (molar ratio=6.2:1) were reacted overHZSM-12 zeolite. The results are summarized below.

                  TABLE XI                                                        ______________________________________                                        REACTION OF 4-OCTENE AND ISOBUTYRIC ACID                                      OVER HZSM-12                                                                                                   C.sub.8 H.sub.17 OBu Isomer                  Reaction                                                                             Tempera-          Yield of                                                                              Distribution                                 Time   ture     Pressure C.sub.8 OBu                                                                           2 / 3 / 4                                    ______________________________________                                        3.1 hr 200° C.                                                                         200 psig 3.8 wt %                                                                              67% 21% 12%                                  4.0 hr 250° C.                                                                         275 psig 7.0 wt %                                                                              50% 32% 18%                                  ______________________________________                                    

EXAMPLE 14

To illustrate reaction of a cycloolefin in the process, a mixture ofnorbornene (bicyclo [2.2.1]-2-heptene) and acetic acid was reacted overHZSM-12 zeolite. The reaction was carried out on a steam bath at about100° C. for 5.5 hours. The addition product, norbornyl acetate, wasformed in about 95% yield.

EXAMPLE 15

Propylene was reacted with acetic acid in the presence of HZSM-12. Intoa 300 cc autoclave were placed 70 ml of glacial acetic acid and 1.0 g ofthe HZSM-12 zeolite. The reactor was heated to 200° C. and liquidpropylene was added at the rate of 10 ml per hour. Samples wereperiodically withdrawn and analyzed. The results are summarized in TableXII. As will be seen from the table, isopropyl acetate was formed inhigh yield with high purity.

                                      TABLE XII                                   __________________________________________________________________________    REACTION OF PROPYLENE WITH ACETIC ACID                                                          Yield of                                                                             Yield of                                                                            Isopropyl                                                        isopropyl                                                                            n-propyl                                                                            acetate,                                       Reaction                                                                           Temperature                                                                           Pressure                                                                           acetate                                                                              acetate                                                                             % of theory                                    __________________________________________________________________________    0.5 hr                                                                             200° C.                                                                        160 psig                                                                            2.9 wt %                                                                            --    36%                                            1.5 hr                                                                             200° C.                                                                        240 psig                                                                            8.0 wt %                                                                            --    34%                                            2.5 hr                                                                             200° C.                                                                        340 psig                                                                           12.8 wt %                                                                            --    36%                                            3.5 hr                                                                             200° C.                                                                        450 psig                                                                           17.0 wt %                                                                            --    36%                                            4.5 hr                                                                             200° C.                                                                        550 psig                                                                           20.5 wt %                                                                            --    35%                                            20.9 hr                                                                            200° C.                                                                        400 psig                                                                           31.6 wt %                                                                            0.03 wt %                                                                           50%                                            __________________________________________________________________________

EXAMPLE 16

Ethylene and acetic acid were reacted in the presence of HZSM-12 in amanner similar to that employed in Example 15. The autoclave, containing1.0 g of HZSM-12 and 70 ml of glacial acetic acid, was heated to 200° C.and then pressurized to 500 psig with ethylene. At 2.5 hours, thetemperature was raised to 250° C. and at 4.0 hours more ethylene wasadded to a pressure of 1000 psig. The level of reaction was low, butethyl acetate was produced as the major reaction product.

Having thus described the present invention with the aid of certainspecific examples thereof, it is to be understood that such examples areintended to be merely illustrative of the disclosed process. Manyvariations thereon may be made without departing from the spirit of thedisclosed invention, as will be evident to those skilled in the art, andsuch variations are intended to come within the scope of the followingclaims:

What is claimed is:
 1. A process for the preparation ofalkyl-carboxylate mixtures enriched in alpha-methylalkyl carboxylateshaving the formula: ##STR3## wherein: R₁ is alkyl of 1 to 10 carbonatoms and R₂ is C₄ to C₁₈ alkyl; said process comprising:reacting anolefin mixture containing at least 25 percent of a C₆ to C₂₀ olefinhaving no unsaturation at the site of the No. 2 carbon atom thereof,with a C₁ to C₁₀ carboxylic acid, said reaction being carried out underester-forming reaction conditions including the presence of a catalystcomprising crystalline zeolite material having a silica to alumina moleratio of at least 12 and a Constraint Index of from about 1 to 12, tothereby selectively produce an alkyl carboxylate ester product which isenriched in the alpha-methylalkyl carboxylate ester.
 2. The process ofclaim 1 wherein said ester-forming reaction conditions include atemperature of between about 25° C. and 600° C. and a pressure of withinthe range of 10⁴ Pa to 10⁷ Pa.
 3. The process of claim 2 wherein saidcarboxylic acid is selected from acetic acid, propionic acid and butyricacid.
 4. The process of claim 1 wherein the olefins in said olefinmixture are linear or slightly branched.
 5. The process of claim 1, 2, 3or 4 wherein said zeolite is selected from the group consisting ofZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48.
 6. The processof claim 5 wherein said zeolite is ZSM-5.
 7. The process of claim 6wherein said ZSM-5 additionally comprises a binder therefor.
 8. Theprocess of claim 5 wherein said zeolite is ZSM-12.
 9. The process ofclaim 8 wherein said ZSM-12 additionally comprises a binder therefor.